The present invention relates generally to exposure apparatuses for fabricating, for example, semiconductor devices, imaging devices, liquid crystal display devices, and more particularly to an exposure apparatus that exposes by using vacuum ultraviolet light like extreme ultraviolet (“EUV”) light and the like.
The reduction projection exposure with UV light has been conventionally used for lithography in manufacturing fine semiconductor devices like semiconductor memories or logic circuits. The transferable minimum critical dimension in the reduction projection exposure is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture of the projection optical system. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a reduction projection optical system using EUV light with a wavelength of 10 to 20 nm shorter than that of the ultraviolet has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
The light absorption in a material remarkably increases in a wave range of the EUV light. Therefore, use of a refractive optical system that is generally used for visible light and UV light is not viable because of its absorption of the EUV light in the optical system. A reflection-type or catoptric optical system is used for an exposure apparatus that uses the EUV light (“EUV exposure apparatus”). A reflection reticle is used instead of a transmission reticle, which forms a pattern to be transferred. The pattern is formed on a mirror by use of an absorber.
The EUV exposure apparatus uses a multilayer mirror or an oblique incidence total reflection mirror as a reflective element. A real part of the refractive index is slightly smaller than 1 in the EUV wave range, and generates total reflection for EUV light incident upon a surface at a large incident angle or an angle close to the reflective surface. An oblique incidence total reflection mirror usually maintains a higher reflectance by several percentages for obliquely incident light within several degrees from the surface, but its degree of freedom in design is small. It is difficult to apply the oblique incidence total reflection mirror to the projection optical system.
A multilayer mirror that alternately forms or layers two kinds of materials having different optical constants is used for a mirror for EUV light with an incident angle close to normal (relatively small) incidence. The multilayer mirror includes, for example, alternately layered molybdenum (Mo) and silicon (Si) on a precisely polished glass plate. For example, a molybdenum layer is about 2 nm thick, and a silicon layer is about 5 nm thick, and about 20 pairs of layers are formed on the glass plate. A sum of thickness of two kinds of materials is generally called a coating cycle. In the above example, the total is 2 nm+5 nm=7 nm.
The multilayer mirror reflects EUV light with a specific wavelength when receiving EUV light. Efficiently reflected EUV light is one within a narrow bandwidth around λ that approximately satisfies Bragg's Equation “2·d·cosθ=λ”, where λ is a wavelength of the reflected EUV light, θ is an incident angle and d is a coating cycle and the bandwidth is about 0.6 to 1 nm. The reflectance of the EUV light would be about 0.7 at most. Non-reflected EUV light is absorbed in the multilayer film or plate, and most of the energy is consumed as heat.
The multilayer mirror has a light loss larger than a mirror for visible light. The number of multilayer mirrors should be maintained minimum when the exposure apparatus uses the multilayer mirrors as the optical system for lithography. In order to realize a large exposure area in the use of minimum number of the mirrors, a scanning exposure method which transfers a large size of area by using a light of arc shaped area (“ring field”) being spaced from an optical axis and scanning the reticle and the wafer simultaneously, is preferable. Therefore, the EUV exposure apparatus generally uses the scanning exposure method.
Here is an explanation about the problem in using conventional exposure apparatus.
EUV light has a property of being absorbed by gas. For example, approximately 50% of the EUV light with the wavelength of 13 nm will be absorbed by air in 1 m distance transmission in air filled space of 10 Pa air pressure. Therefore, the air pressure of the space where the EUV light will be transmitted should be less than 10−1 Pa or preferably be less than 10−3 Pa for preventing the absorption by gas.
Moreover, in case of carbon-containing molecule such as a hydrocarbon remaining in the space where the optical element has been arranged, carbon gradually adheres on the surface of the optical element by exposure of EUV light. And it has been a problem of the reflectance decrease on EUV light absorption caused by adhered carbon. The air pressure of the space arranging the optical element to be exposed by EUV light should be less than 10−4 Pa or preferably be less than 10−6 Pa to prevent carbon adhering.
Since EUV light with the wavelength of 13–14 nm has a property of being absorbed largely by transmitting the lens or in the air, the EUV exposure apparatus does not use a conventional transmission reticle but reflection reticle, and should arrange all optical elements used in an illumination optical system or the projection optical system in vacuum state.
The reflection reticle forms information of a circuit pattern according to the difference of the reflected EUV light intensity between on a light-reflecting portion and on a light-absorbing portion. Heat generation of the reflection reticle for EUV light becomes large in comparison with the conventional transmission reticle in receiving the illumination light because of partial absorption of the reflecting-type reticle. Moreover, the reflecting-type reticle is located in vacuum state, and the heat radiation to atmosphere is little. The efficiency of the heat radiation is low because the reticle only radiates the heat by conduction to a reticle chuck which holds the reticle.
A method for holding the reticle by the reticle chuck uses an electrostatic suction instead of a conventional vacuum suction for the vacuum state of the atmosphere. The following problem has been caused by this electrostatic suction in comparison with the conventional suction. Materials of the reticle chuck are limited to acquire enough force of electrostatic suction. It does not result that the reticle can always use material having ideal low coefficient of thermal expansion.
A leakage current as a heat source generated at an electrostatic suction portion slightly happens to heat the electrostatic portion. This causes a slight heat expansion of the reticle chuck in the EUV exposure apparatus. A holding surface of the reticle chuck has an area same as or larger than the reticle for holding the reticle in its whole surface. The reticle has a space for information for positioning and for gripping by the conveying apparatus, peripheral of the circuit pattern portion. So, the whole area of the reticle is larger than the area actually used for exposure.
The EUV exposure apparatus generally uses scanning exposure method as explained before. The laser interferometer measures the position of the reticle and the wafer. FIGS. 7A and 7B show an example of the position measurement of the reticle in the conventional exposure apparatus which is for non-EUV light. The position of the reticle 101 is acquired by measuring the position of a reference surface 103 as a measuring point (measuring surface) which is provided in the reticle chuck 102 for holding the reticle 101, as shown in FIGS. 7A and 7B.
This reference surface 103 is provided on a shorter side 102a of the reticle chuck 102 to measure the position in sub-scanning direction of the reticle chuck 102. Another reference surface (not shown) is also provided on a longer side 102b of the reticle chuck 102 to measure the position in main-scanning direction of the reticle chuck 102. The sub-scanning direction in the reticle surface or the wafer surface defines X-direction. The main-scanning direction that is perpendicular to X-direction in the reticle surface or the wafer surface defines Y-direction. Direction perpendicular to the reticle surface or the wafer surface defines Z-direction.
The reticle hardly absorbs the light and hardly generates heat in the conventional exposure apparatus. Even in the case of a little heat generation, it has not been a big problem because the heat has been radiated and diffused into circumferential air. And more, the reticle chuck 101 has not generated heat by its leakage current for being vacuum suction type
However, the reflection reticle absorbs the light and generates heat in the EUV exposure apparatus. And the reticle chuck of electrostatic suction type also generates heat by its leakage current. Moreover, being arranged in vacuum state, the reticle and the reticle chuck hardly diffuses heat, lack of cooling effect and become high temperature.
In this case, the reticle 101 and the reticle chuck 102 expand by thermal expansion in the conventional structure shown in FIGS. 7A and 7B, and the reticle 101 and the reference surface 103 shift their positions. For example, if the reticle 101 and the reference surface 103 are spaced as shown in FIGS. 7A and 7B, the position of the reticle 101 is measured with error, because position shift of the reticle 101 and the reference surface 103 by the thermal expansion are different from each other and the position of the reticle 101 does not correspond to that of the reference surface 103 by temperature change. This measurement error causes imprecise transcription of the circuit pattern on the reticle to the wafer in exposure by scanning a reticle stage 104, and the problem of high fraction defective of a chip. The material of the reticle chuck cannot always have the coefficient of thermal expansion for using electrostatic suction type as the reticle chuck, and it makes the problem about thermal expansion big.